The Last Enchantment (Arthurian Saga Series #3)

Arthur Pendragon is King! Unchallenged on the battlefield, he melds the country together in a time of promise. But sinister powers plot to destroy Camelot, and when the witch-queen Morgause — Arthur's own half sister — ensnares him in an incestuous liaison, a fatal web of love, betrayal, and bloody vengeance is woven.